Right... i'm hoping to explain this as clear as I possibly can!
Now... the use of "Phasing In" is probably not the best use of words because that suggests there is some form of Cross Over! BUT, I have had it confirmed that this is NOT the case.
And so.... i'm going to explain it in 6 Cycles using colour co-ordination!!
As you can see, the ability to Craft
an Artifact for a Base will occur 6wks after it's finished it's run in The Spire.
Once the date has been confirmed for the next Cycle, both the previous Base and Artifact will no longer be available after that date.
Some Evolving Buildings and Artifacts (such as the original Phoenix and Bears) are constantly available in the Crafting Rotations and are not subject to any specific periods of time.
Please also note that if a Player already has a Base in their City or Inventory this will not show in their Crafting Menu.
I hope this helps you to understand this a little better!!